In situ exfoliation method to fabricate a graphene-reinforced polymer matrix composite

ABSTRACT

A method for forming a graphene-reinforced-polymer matrix composite by distributing graphite microparticles into a molten thermoplastic polymer phase comprising one or more molten thermoplastic polymers; and applying a succession of shear strain events to the molten polymer phase so that the molten polymer phase exfoliates the graphene successively with each event, until tearing of exfoliated multilayer graphene sheets occurs arid produces reactive edges on the multilayer sheets that react with and cross-link the one or more thermoplastic polymers; where the one or more thermoplastic polymers are selected from thermoplastic polymers subject to UV degradation.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. National Phase of InternationalApplication No. PCT/US2014/034624, filed Apr. 18, 2014, which claims thebenefit of priority under 35 U.S.C. §119(e) of U.S. ProvisionalApplication No. 61/813,621, filed on Apr. 18, 2013, the entiredisclosures of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to high efficiency mixing methods totransform a polymer composite containing well-crystallized graphiteparticles into nano-dispersed single or multi-layer graphene particleshaving various commercial applications. The present invention alsorelates to methods to activate graphite and graphene using in situmechanical exfoliation.

BACKGROUND OF THE INVENTION

Polymer compositions are being increasingly used in a wide range ofareas that have traditionally employed the use of other materials, suchas metals. Polymers possess a number of desirable physical properties,are light weight, and inexpensive. In addition, many polymer materialsmay be formed into a number of various shapes and forms and exhibitsignificant flexibility in the forms that they assume, and may be usedas coatings, dispersions, extrusion and molding resins, pastes, powders,and the like.

There are various applications for which it would be desirable to usepolymer compositions, which require materials with electricalconductivity. However, a significant number of polymeric materials failto be intrinsically electrically or thermally conductive enough for manyof these applications.

Graphene is a substance composed of pure carbon in which atoms arepositioned in a hexagonal pattern in a densely packed one-atom thicksheet. This structure is the basis for understanding the properties ofmany carbon-based materials, including graphite, large fullerenes,nano-tubes, and the like (e.g., carbon nano-tubes are generally thoughtof as graphene sheets rolled up into nanometer-sized cylinders).Graphene is a single planar sheet of sp² bonded carbon atoms. Grapheneis not an allotrope of carbon because the sheet is of finite size andother elements can be attached at the edge in non-vanishingstoichiometric ratios.

When used to reinforce polymers, graphene in any form increases polymertoughness by inhibiting crack propagation. Graphene can also be added topolymers and other compositions to provide electrical and thermalconductivity. The thermal conductivity of graphene makes it an idealadditive for thermal management (e.g., planar heat dissipation) forelectronic devices and lasers. Some commercial applications of carbonfiber-reinforced polymer matrix composites (CF-PMCs) include aircraftand aerospace systems, automotive systems and vehicles, electronics,government defense/security, pressure vessels, and reactor chambers,among others.

Progress in the development of low cost methods to effectively producegraphene-reinforced polymer matrix composites (G-PMCs) remains veryslow. Currently, some of the challenges that exist affecting thedevelopment of G-PMCs viable for use in real world applications includethe expense of the materials and the impracticality of the presentlyused chemical and/or mechanical manipulations for large-scale commercialproduction. It would thus be desirable for a low cost method to producea G-PMC suitable for large-scale commercial production that offers manyproperty advantages, including increased specific stiffness andstrength, enhanced electrical/thermal conductivity, and retention ofoptical transparency.

SUMMARY OF THE INVENTION

The present disclosure provides polymer processing methods to fabricatea graphene-reinforced polymer matrix composite (G-PMC) by elongationalflow and folding of well-crystallized graphite particles dispersed in amolten polymer matrix.

In one aspect, there is provided herein a method for forming agraphene-reinforced polymer matrix composite, including: distributinggraphite microparticles into a molten thermoplastic polymer phase; andapplying a succession of shear strain events to the molten polymer phaseso that the molten polymer phase exfoliates the graphite successivelywith each event until at least 50% of the graphite is exfoliated to forma distribution in the molten polymer phase of single- and multi-layergraphene nano-particles less than 50 nanometers thick along a c-axisdirection.

In certain embodiments, the graphite particles may be prepared bycrushing and grinding a graphite-containing mineral to millimeter-sizeddimensions.

In certain embodiments, the millimeter-sized particles may be reduced tomicron-sized dimensions using any known method, such as ball milling orattritor milling.

In certain embodiments, the graphite particles are extracted from themicron-sized particle mixture, preferably by a flotation method.

In certain embodiments, the extracted graphite particles may beincorporated in a polymer matrix using a single screw extruder withaxial fluted extensional mixing elements or spiral fluted extensionalmixing elements.

In certain embodiments, the graphite-containing polymer matrix issubjected to repeated extrusion to induce exfoliation of the graphiticmaterial, thus forming a uniform dispersion of graphene nanoparticles inthe polymer matrix.

In certain embodiments, the thermoplastic polymer is an aromaticpolymer. The aromatic polymer preferably comprises phenyl groups,optionally substituted, either as part of the backbone or assubstituents on the backbone. In certain embodiments the optionallysubstituted phenyl groups are contained within the polymer backbone asoptionally substituted phenylene groups. In certain other embodimentsthe optionally substituted phenyl groups are substituents on thepolymer. In specific embodiments, the thermoplastic polymer is selectedfrom polyetheretherketones, polyether-ketones, polyphenylene sulfides,polyethylene sulfides, polyetherimides, polyvinylidene fluorides,polysulfones, polycarbonates, polyphenylene ethers or oxides, polyamidessuch as nylons, aromatic thermoplastic polyesters, aromaticpolysulfones, thermoplastic polyimides, liquid crystal polymers,thermoplastic elastomers, polyethylenes, polypropylenes, polystyrene,acrylics, such as polymethylmethacrylate, polyacrylonitrile,acrylonitrile butadiene styrene, and the like,ultra-high-molecular-weight polyethylene, polytetrafluoroethylene,polyoxymethylene plastic, polyaryletherketones, polyvinylchloride, andmixtures thereof.

In certain embodiments, in combination with other embodiments, thesuccession of shear strain events may be applied until at least 50% ofthe graphite is exfoliated to form a distribution in the molten polymerphase of single- and multi-layer graphene nanoparticles less than 25nanometers thick along the c-axis direction.

In certain embodiments, in combination with other embodiments, thesuccession of shear strain events may be applied until at least 50% ofthe graphite is exfoliated to form a distribution in the molten polymerphase of single- and multi-layer graphene nanoparticles less than 10nanometers thick along the c-axis direction.

In certain embodiments, in combination with other embodiments, thesuccession of shear strain events may be applied until at least 90% ofthe graphite is exfoliated to form a distribution in the molten polymerphase of single- and multi-layer graphene nanoparticles less than 10nanometers thick along the c-axis direction.

In certain embodiments, in combination with other embodiments, thesuccession of shear strain events may be applied until at least 80% ofthe graphite is exfoliated to form a distribution in the molten polymerphase of single- and multi-layer graphene nanoparticles less than 10nanometers thick along the c-axis direction.

In certain embodiments, in combination with other embodiments, thesuccession of shear strain events may be applied until at least 75% ofthe graphite is exfoliated to form a distribution in the molten polymerphase of single- and multi-layer graphene nanoparticles less than 10nanometers thick along the c-axis direction.

In certain embodiments, in combination with other embodiments, thesuccession of shear strain events may be applied until at least 70% ofthe graphite is exfoliated to form a distribution in the molten polymerphase of single- and multi-layer graphene nanoparticles less than 10nanometers thick along the c-axis direction.

In certain embodiments, in combination with other embodiments, thesuccession of shear strain events may be applied until at least 60% ofthe graphite is exfoliated to form a distribution in the molten polymerphase of single- and multi-layer graphene nanoparticles less than 10nanometers thick along the c-axis direction.

In certain embodiments, in combination with other embodiments, thegraphite may be doped with other elements to modify the surfacechemistry of the exfoliated graphene nanoparticles.

In certain embodiments, in combination with other embodiments, thegraphite is expanded graphite.

In certain embodiments, in combination with other embodiments, thesurface chemistry or nanostructure of the dispersed graphite may bemodified to enhance bond strength with the polymer matrix to increasestrength and stiffness of the graphene composite.

In certain embodiments, in combination with other embodiments,directional alignment of the graphene nanoparticles is used to obtainone-, two- or three-dimensional reinforcement of the polymer matrixphase.

In another aspect of the disclosed invention, there is provided herein amethod for forming a cross-linked G-PMC, including: distributinggraphite microparticles into a molten thermoplastic polymer phasecomprising one or more molten thermoplastic polymers; and applying asuccession of shear events to said molten polymer phase, so that saidmolten polymer phase exfoliates the grapheme with each event, untiltearing of exfoliated multilayer graphene sheets occurs and producesreactive edges on said multilayer sheets that react with and cross-linksaid thermoplastic polymer.

In another aspect of the disclosed invention, there is provided herein amethod for forming a high strength cross-linked G-PMC, including:distributing graphite microparticles into a molten thermoplastic polymerphase comprising one or more molten thermoplastic polymers; applying asuccession of shear strain events to the molten polymer phase so thatsaid molten polymer phase exfoliates the graphene successively with eachevent, until tearing of exfoliated multilayer graphene sheets occurs andproduces reactive edges on said multilayer sheets that react with andcross-link said thermoplastic polymer, to form a graphene-reinforcedpolymer matrix composite; and further grinding and distributing thegraphene-reinforced polymer matrix composite with anothernon-cross-linked thermoplastic polymer.

In certain embodiments, the graphite particles may be prepared bycrushing and grinding a graphite-containing mineral to millimeter-sizeddimensions, followed by reduction to micron-sized particles by milling.

In certain embodiments, the graphite particles are extracted from themicron-sized particle mixture, preferably by a flotation method, toobtain Separated Mineral Graphite (“SMG”).

In certain embodiments, the molten thermoplastic polymer phase comprisestwo molten thermoplastic polymers.

In certain embodiments, the thermoplastic polymers are selected frompolyether-etherketone (PEEK), polyetherketone (PEK), polyphenylenesulfide (PPS), polyethylene sulfide (PES), polyetherimide (PEI),polyvinylidene fluoride (PVDF), polycarbonate (PC), poly-phenyleneether, aromatic thermoplastic polyesters, thermoplastic polyimides,liquid crystal polymers, thermoplastic elastomers, polyethylene,polypropylene, polystyrene (PS), acrylics, such aspolymethylmethacrylate (PMMA), polyacrylonitrile (PAN), acrylonitrilebutadiene styrene (ABS), and the like, ultra-high-molecular-weightpolyethylene (UHMWPE), polytetra-fluoroethylene (PTFE/Teflon®),polyamides (PA) such as nylons, polyphenylene oxide (PPO),polyoxymethylene plastic (POM/Acetal), polyaryletherketones,polyvinylchloride (PVC), and mixtures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C, 1D, 1E, 1F, 1G, 1H, 1J and 1I illustrate themorphology analysis of 2% graphite exfoliated in polysulfone at mixingtimes of 3 minutes, 30 minutes, and 90 minutes according to an in situexfoliation method of the present disclosure.

FIGS. 2A, 2B and 2C illustrate micrographs of 90G-PMC at various scalesand magnification levels according to an in situ exfoliation method ofthe present disclosure.

FIGS. 3A, 3B, 3C and 3D illustrate the morphology of SMG-PEEK 90 at (a)10 μm scale and 1,000×, (b) 10 μm scale and 5,000×, (c) 1 μm scale and10,000×, and (d) 1 μm scale and 50,000×.

DETAILED DESCRIPTION OF THE INVENTION

This disclosure is not limited to the particular systems, methodologiesor protocols described, as these may vary. The terminology used in thisdescription is for the purpose of describing the particular versions orembodiments only, and is not intended to limit the scope.

As used in this document, the singular forms “a,” “an,” and “the”include plural reference unless the context clearly dictates otherwise.Unless defined otherwise, all technical and scientific terms used hereinhave the same meanings as commonly understood by one of ordinary skillin the art. All publications mentioned in this document are incorporatedby reference. All sizes recited in this document are by way of exampleonly, and the invention is not limited to structures having the specificsizes or dimensions recited below. Nothing in this document is to beconstrued as an admission that the embodiments described in thisdocument are not entitled to antedate such disclosure by virtue of priorinvention. As used herein, the term “comprising” means “including, butnot limited to.”

The following term(s) shall have, for purposes of this application, therespective meanings set forth below:

The term “graphene” refers to the name given to a single layer of carbonatoms densely packed into a fused benzene-ring structure. Graphene, whenused alone, may refer to multi-layer graphene, graphene flakes, grapheneplatelets, and few-layer graphene or single-layer graphene in a pure anduncontaminated form.

The present invention provides a high efficiency mixing method totransform a polymer composite that contains well-crystallized graphiteparticles into nano-dispersed single- or multi-layer graphene particles.The method involves in situ exfoliation of the graphite layers bycompounding in a batch mixer or extruder that imparts repetitive, highshear strain rates. In both processes, longer mixing times provideenhanced exfoliation of the graphite into graphene nanoparticles withinthe polymer matrix composite (PMC). In addition, additives may be usedto promote sufficient graphene/polymer bonding, thereby yielding a lowdensity graphene-reinforced polymer matrix composite (G-PMC). The methodis low cost to produce a G-PMC that offers numerous property advantages,including increased specific stiffness and strength, enhancedelectrical/thermal conductivity, and retention of optical transparency.Furthermore, these properties are tunable by modification of theprocess, vide infra.

Repeated compounding during a batch mixing process or single screwextrusion is used to progressively transform the initialgraphite-particle dispersion into a uniform nano-dispersion of discretegraphene nanoparticles. In some cases, an inert gas or vacuum may beused during processing. The method is described herein as “mechanical”exfoliation to distinguish it from “chemical” exfoliation, which is theprimary thrust of much of the current research. An advantage of themechanical method is that contamination-free graphene-polymer interfacesare formed during high-shear mixing, thus ensuring good interfaceadhesion or bonding. Other advantages of in situ exfoliation are that itavoids making and handling graphene flakes, as well as avoiding the needto disperse them uniformly in the polymer matrix phase. Superior mixingproduces finer composite structures and very good particle distribution.

Depending on the number of in situ shear strain events, the methodprovides multi-layer graphene, graphene flakes, graphene platelets,few-layer graphene or single-layer graphene in a pure and uncontaminatedform. Platelets have diamond-like stiffness and are used for polymerreinforcement. Graphene in any form increases polymer toughness byinhibiting crack propagation as a reinforcement for polymers. Graphenemay be used as an additive to polymers and other compositions to provideelectrical and thermal conductivity. The thermal conductivity ofgraphene makes it a desirable additive for thermal management forelectronic devices and lasers.

Graphite, the starting material from which graphene is formed, iscomposed of a layered planar structure in which the carbon atoms in eachlayer are arranged in a hexagonal lattice. The planar layers are definedas having an “a” and a “b” axis, with a “c” axis normal to the planede-fined by the “a” and “b” axes. The graphene particles produced by theinventive method have an aspect ratio defined by the “a” or “b” axisdistance divided by the “c” axis distance. Aspect ratio values for theinventive nanoparticles exceed 25:1 and typically range between 50:1 and1000:1.

It should be understood that essentially any polymer inert to graphiteand capable of imparting sufficient shear strain to exfoliate graphenefrom the graphite may be used in the method of the present invention.Examples of such polymers include, but are not limited to,poly-etheretherketones (PEEK), polyetherketones (PEK), polyphenylenesulfides (PPS), polyethylene sulfide (PES), polyetherimides (PEI),polyvinylidene fluoride (PVDF), polysulfones (PSU), polycarbonates (PC),polyphenylene ethers, aromatic thermoplastic polyesters, aromaticpolysulfones, thermoplastic polyimides, liquid crystal polymers,thermoplastic elastomers, polyethylene, poly-propylene, polystyrene(PS), acrylics, such as polymethylmethacrylate (PMMA),polyacrylo-nitrile (PAN), acrylonitrile butadiene styrene (ABS), and thelike, ultra-high-molecular-weight polyethylene (UHMWPE),polytetrafluoroethylene (PTFE/Teflon®), polyamides (PA) such as nylons,polyphenylene oxide (PPO), polyoxymethylene plastic (POM/Acetal),polyarylether-ketones, polyvinylchloride (PVC), mixtures thereof, andthe like. Polymers capable of wetting the graphite surface may be usedas well as high melting point, amorphous polymers in accordance with themethod of the present invention. In certain embodiments, thethermoplastic polymer of the graphene-reinforced polymer matrix is anaromatic polymer, as defined herein.

The graphene may be produced as a graphene-polymer mixture suitable foruse as-is as a G-PMC that can be pelletized by conventional means forsubsequent fabrication processing. Alternatively, higher concentrationsof graphite may be used at the outset to provide a graphene-polymermasterbatch in concentrated form that can also be pelletized and thenused to add graphene to polymer compositions as a reinforcing agent. Asa further alternative, the graphene may be separated from the polymer,for example, by combustion or selective dissolution, to provideessentially pure particles of graphene.

Graphene-reinforced polymers according to the present inventiontypically contain between about 0.1 and about 30 wt % graphene. Moretypically, the polymers contain between about 1.0 and about 10 wt %graphene. Polymer masterbatches typically contain between about 5 andabout 50 wt % graphene, and more typically between about 10 and about 30wt % graphene.

The availability of graphite-rich mineral deposits, containingrelatively high concentrations (e.g., about 20%) of well-crystallizedgraphite, makes for a low cost and virtually inexhaustible source of rawmaterial. As discussed below, the extraction of graphite particles frommined material can be accomplished in a cost-effective manner. Syntheticgraphite of high purity and exceptional crystallinity (e.g., pyrolyticgraphite) may also be used for the same purpose. However, in this case,the batch mixing or extrusion compounding-induced exfoliation processcreates a laminated composite, in which the graphene nanoparticles areoriented over a relatively large area. Such laminated composites may bepreferred for specific applications.

Mechanical exfoliation of graphite within a polymer matrix may beaccomplished by a polymer processing technique that imparts repetitivehigh shear strain events to mechanically exfoliate graphitemicroparticles into multi- or single-layer graphene nanoparticles withinthe polymer matrix.

For purposes of the present invention, graphite micro-particles aredefined as graphite in which at least 50% of the graphite consists ofmultilayer graphite crystals ranging between 1.0 and 1000 microns thickalong the c-axis of the lattice structure. Typically 75% of the graphiteconsists of crystals ranging between 100 and 750 microns thick. Expandedgraphite may also be used. Expanded graphite is made by forcing thecrystal lattice planes apart in natural flake graphite, thus expandingthe graphite, for example, by immersing flake graphite in an acid bathof chromic acid, then concentrated sulfuric acid. Expanded graphitesuitable for use in the present invention include expanded graphite withopened edges at the bilayer level, such as MESOGRAF.

A succession of shear strain events is defined as subjecting the moltenpolymer to an alternating series of higher and lower shear strain ratesover essentially the same time intervals so that a pulsating series ofhigher and lower shear forces associated with the shear strain rate areapplied to the graphite particles in the molten polymer. Higher andlower shear strain rates are defined as a first higher, shear strainrate that is at least twice the magnitude of a second lower shear strainrate. The first shear strain rate will range between 100 and 10,000sec⁻¹. At least 1,000 to over 10,000,000 alternating pulses of higherand lower shear strain pulses are applied to the molten polymer to formthe exfoliated graphene nanoparticles. The number of alternating pulsesrequired to exfoliate graphite particles into graphene particles may bedependent on the original graphite particle dimensions at the beginningof this process, i.e., smaller original graphite particles may need alower number of alternating pulses to achieve graphene than largeroriginal graphite particles. This can be readily determined by one ofordinary skill in the art guided by the present specification withoutundue experimentation.

After high-shear mixing, the graphene flakes are uniformly dispersed inthe molten polymer, are randomly oriented, and have high aspect ratio.Orientation of the graphene may be achieved by many different methods.Conventional drawing, rolling, and extrusion methods may be used todirectionally align the graphene within the PMC fiber, filament, ribbon,sheet, or any other long-aspect shape. The method to fabricate andcharacterize a G-PMC is comprised of four main steps comprising:

-   -   1. Extraction of crystalline graphite particles from a mineral        source;    -   2. Incorporation of the extracted graphite particles into a        polymer matrix phase and conversion of the graphite-containing        polymer into a graphene-reinforced polymer matrix composite        (G-PMC) by a high efficiency mixing/exfoliation process;    -   3. Morphology analysis to determine the extent of mechanical        exfoliation and distribution of multi-layer graphene and        graphene nanoparticles; and    -   4. X-ray diffraction analysis to determine multi-layer graphene        or graphene crystal size as a function of mechanical        exfoliation.

Highly crystalline graphite may be extracted from graphite ore by amulti-step process, as described below.

-   -   1. Crushing: A drilled rod of graphite ore from the mine may be        placed in a vice and crushed.    -   2. Grinding: The crushed graphite ore may be then ground by        mortar and pestle.    -   3. Size Reduction: The ground graphite ore may be placed in a        sieve with a 1-mm mesh size and size reduced. Larger pieces that        do not pass through the screen may be ground by mortar and        pestle and then size reduced through the 1-mm mesh size again.        Eventually, all of the material passed through the 1-mm mesh        size to obtain graphite ore powder.    -   4. Density Separation by Water: The 1-mm sized powder may be        placed in a column filled with water and agitated until a clear        separation formed between the more dense portions of the solids        and the less dense portions. Graphite is near the density of        water (1 g/cm³), while silicon is much more dense (2.33 g/cm³).        The uppermost materials are siphoned off with the water and then        dried. The dried powder graphite is referred to as Separated        Mineral Graphite (SMG).

In commercial practice, very large crushing and grinding machines areavailable to produce tonnage quantities of mixed powders, from which thegraphite component can be separated by standard floatation methods.

One embodiment is directed to an in situ exfoliation method offabricating a G-PMC. In this method, a polymer that is uniformly blendedwith micron-sized crystalline graphite particles is subjected torepeated compounding-element processing during batch mixing or extrusionat a temperature where the polymer adheres to the graphite particles.Typical polymers have a heat viscosity (without graphite) greater than100 cps at the compounding temperature. The compounding temperature willvary with the polymer and can range between room temperature (forpolymers that are molten at room temperature) and 600° C. Typicalcompounding temperatures will range between 180° C. and 400° C.

In one embodiment, the extrusion compounding elements are as describedin U.S. Pat. No. 6,962,431, the disclosure of which is incorporatedherein by reference, with compounding sections, known as axial flutedextensional mixing elements or spiral fluted extensional mixingelements. The compounding sections act to elongate the flow of thepolymer and graphite, followed by repeated folding and stretching of thematerial. This results in superior distributive mixing, which in turn,causes progressive exfoliation of the graphite particles into discretegraphene nanoparticles. Batch mixers may also be equipped withequivalent mixing elements. In another embodiment, a standard-typeinjection molding machine is modified to replace the standard screw witha compounding screw for the purpose of compounding materials as thecomposition is injection molded. Such a device is disclosed in US2013/0072627, the entire disclosure of which is incorporated herein byreference.

Thus, the effect of each compounding pass is to shear-off graphenelayers one after the other, such that the original graphite particlesare gradually transformed into a very large number of graphenenanoparticles. After an appropriate number of such passes, the finalresult is a uniform dispersion of discrete graphene nanoparticles in thepolymer matrix phase. Longer mixing times or a higher number of passesthrough the compounding elements provides smaller graphite crystal sizeand enhanced exfoliation of graphite into graphene nanoparticles withinthe polymer matrix; however, the shear events should not be of aduration that would degrade the polymer.

As the content of graphene nanoparticles increases during multi-passextrusion, the viscosity of the polymer matrix increases due to theinfluence of the growing number of polymer/graphene interfaces. Toensure continued refinement of the composite structure, the extrusionparameters are adjusted to compensate for the higher viscosity of thecomposite.

Automated extrusion systems are available to subject the compositematerial to as many passes as desired, with mixing elements as describedin U.S. Pat. No. 6,962,431, and equipped with a re-circulating stream todirect the flow back to the extruder input. Since processing of thegraphene-reinforced PMC is direct and involves no handling of grapheneparticles, fabrication costs are low.

In order to mechanically exfoliate graphite into multi-layer grapheneand/or single-layer graphene, the shear strain rate generated in thepolymer during processing must cause a shear stress in the graphiteparticles greater than the critical stress required to separate twolayers of graphite, or the interlayer shear strength (ISS). The shearstrain rate within the polymer is controlled by the type of polymer andthe processing parameters, including the geometry of the mixer,processing temperature, and speed in revolutions per minute (RPM).

The required processing temperature and speed (RPM) for a particularpolymer is determinable from polymer rheology data given that, at aconstant temperature, the shear strain rate ({dot over (γ)}) is linearlydependent upon RPM, as shown by Equation 1. The geometry of the mixerappears as the rotor radius, r, and the space between the rotor and thebarrel, Δr.

$\begin{matrix}{\overset{.}{\gamma} = {\left( \frac{2\pi \; r}{\Delta \; r} \right)\left( \frac{RPM}{60} \right)}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

Polymer rheology data collected for a particular polymer at threedifferent temperatures provides a log shear stress versus log shearstrain rate graph. The ISS of graphite ranges between 0.2 MPa and 7 GPa,but a new method has quantified the ISS at 0.14 GPa. Thus, tomechanically exfoliate graphite in a polymer matrix during processing,the required processing temperature, shear strain rate, and RPM isdeterminable for a particular polymer from a graph of the log shearstress versus the log shear strain rate, collected for a polymer at aconstant temperature, so that the shear stress within the polymer isequal to or greater than the ISS of graphite. Under typical processingconditions, polymers have sufficient surface energy to behave like thesticky side of adhesive tape, and thus are able to share the shearstress between the polymer melt and the graphite particles.

In one embodiment, a method for forming a G-PMC includes distributinggraphite microparticles into a molten thermoplastic polymer phase. Asuccession of shear strain events are then applied to the molten polymerphase so that the molten polymer phase exfoliates the graphitesuccessively with each event until at least 50% of the graphite isexfoliated to form a distribution in the molten polymer phase of single-and multi-layer graphene nanoparticles less than 50 nanometers thickalong a c-axis direction.

In another embodiment, a method for forming a cross-linked G-PMCincludes distributing graphite microparticles into a moltenthermoplastic polymer phase comprising one or more molten thermoplasticpolymers. A succession of shear strain events, as illustrated in theexamples, are then applied to the molten polymer phase so that themolten polymer phase exfoliates the graphene successively with eachevent until a lower level of graphene layer thickness is achieved, afterwhich point ripping and tearing of exfoliated multilayer graphene sheetsoccurs and produces reactive edges on the multilayer sheets that reactwith and cross-link the thermoplastic polymer.

In another embodiment, the cross-linked G-PMC can be ground intoparticles and blended with non-cross-linked host polymers to serve astoughening agents for the host polymer. The non-cross-linked polymeracquires the properties of the cross-linked polymer because of chainentanglement between the two polymer species. The present inventiontherefore also includes cross-linked polymers of the present inventionin particulate form that can be blended with other polymers to form ahigh strength composite. In one embodiment cross-linked polystyrene andpolymethyl methacrylate (PMMA) particles of the present invention can beused as toughening agents for host polymers. Compositions according tothe present invention include host thermoplastic polymers toughened withbetween about 1 and about 75% by weight of the cross-linked polymerparticles of the present invention. In one embodiment, the host polymersare toughened with between about 10 and about 50% by weight of thecross-linked polymer particles.

In certain embodiments, the thermoplastic polymer is an aromaticpolymer. As defined herein the term “aromatic polymer” refers to apolymer comprising aromatic moieties, either as part of the polymerbackbone or as substituents attached to the polymer backbone, optionallyvia a linker. Linkers include linear or branched alkylene groups, suchas methylene, ethylene, and propylene, linear or branched heteroalkylenegroups, such as —OCH₂—, —CH₂O—, —OCH₂CH₂—, —CH₂CH₂O—, —OCH₂CH₂CH₂—,—CH₂OCH₂—, —OCH(CH₃)—, —SCH₂—, —CH₂S—, —NRCH₂—, —CH₂NR—, and the like,where the heteroatom is selected from the groups consisting of oxygen,nitrogen and sulfur, and R is selected from hydrogen and lower alkyl.Linkers can also be heteroatomic, such as —O—, —NR— and —S—. When thelinkers contain sulfur, the sulfur atom is optionally oxidized. Thearomatic moieties are selected from monocyclic, e.g. phenyl, andpolycyclic moieties, e.g. naphthyl, indole, anthracene, etc., and areoptionally substituted with amino, NHR, NR₂, halogen, nitro, cyano,alkylthio, alkoxy, alkyl, haloalkyl, CO₂R where R is defined as above,and combinations of two or more thereof. The aromatic moieties can alsobe heteroaryl, comprising one to three heteroatoms selected from thegroup consisting of oxygen, nitrogen and sulfur, and optionallysubstituted as described above. The aromatic polymer preferablycomprises phenyl groups, optionally substituted as disclosed above,either as part of the polymer backbone or as substituents on thebackbone, the latter optionally through a linker, as disclosed above. Incertain embodiments the optionally substituted phenyl groups arecontained within the polymer backbone as optionally substitutedphenylene groups. In certain other embodiments the optionallysubstituted phenyl groups are substituents on the polymer backbone,optionally connected through a linker, as described above.

Examples of thermoplastic host polymers include, but are not limited to,polyetherether-ketone (PEEK), polyetherketone (PEK), polyphenylenesulfide (PPS), polyethylene sulfide (PES), polyetherimide (PEI),polyvinylidene fluoride (PVDF), polysulfone (PSU), polycarbonate (PC),polyphenylene ether, aromatic thermoplastic polyesters, aromaticpolysulfones, thermo-plastic polyimides, liquid crystal polymers,thermoplastic elastomers, polyethylene, poly-propylene, polystyrene(PS), acrylics such as polymethylmethacrylate (PMMA), polyacrylo-nitrile(PAN), acrylonitrile butadiene styrene (ABS), and the like,ultra-high-molecular-weight polyethylene (UHMWPE),polytetrafluoroethylene (PTFE/Teflon®), polyamides (PA) such as nylons,polyphenylene oxide (PPO), polyoxymethylene plastic (POM/Acetal),polyimides, polyaryletherketones, polyvinylchloride (PVC), acrylics,mixtures thereof, and the like. When the thermoplastic host polymer andthe cross-linked polymer are the same polymer species, the cross-linkedpolymer particles are essentially a concentrated masterbatch of thedegree of cross-linked species desired to be introduced to the polymerformulation.

Therefore, another aspect of the present invention provides a method forforming a high strength graphene-reinforced polymer matrix composite bydistributing graphite micro-particles into a molten thermoplasticpolymer phase comprising one or more molten thermoplastic polymers. Asuccession of shear strain events, as illustrated in the examples, arethen applied to the molten polymer phase so that the molten polymerphase exfoliates the graphene successively with each event until tearingof exfoliated multilayer graphene sheets occurs and produces reactiveedges on said multilayer sheets that react with and cross-link thethermoplastic polymer. The cross-linked graphene and thermoplasticpolymer is then ground into particles that are distributed into anothernon-cross-linked polymer.

Thus, activated graphene is formed as the graphene fractures acrossbasal plane and offers potential sites for cross-linking to the matrixor attaching other chemically unstable groups for functionalization.Therefore, the cross-linking is performed under exclusion of oxygen,preferably under an inert atmosphere or a vacuum, so that the reactiveedges do not oxidize or otherwise become unreactive. Forming covalentbonds between graphene and the matrix significantly increases thecomposite strength. Polymers that cross-link when subjected to themethod of the present invention include polymers subject to degradationby ultraviolet (UV) light. This includes polymers containing aromatic,e.g., benzene rings, such as polystyrene, polymers containing tertiarycarbons, such as polypropylene and the like, polymers containingbackbone oxygens, such as poly(alkylene oxides), and the like.

In certain embodiments, the graphite particles may be prepared bycrushing and grinding a graphite-containing mineral to millimeter-sizeddimensions. The millimeter-sized particles may be reduced tomicron-sized dimensions using ball milling and attritor milling.

In certain embodiments, the graphite particles may be extracted from themicron-sized particle mixture, preferably by a flotation method. Theextracted graphite particles may be incorporated in a polymer matrixusing a single screw extruder with axial fluted extensional mixingelements or spiral fluted extensional mixing elements. Thegraphite-containing polymer matrix is subjected to repeated extrusion asdescribed herein to induce exfoliation of the graphitic material, thusforming a uniform dispersion of graphene nanoparticles in the polymermatrix.

In other embodiments, the succession of shear strain events may beapplied until at least 50% of the graphite is exfoliated to form adistribution in the molten polymer phase of single- and multi-layergraphene nanoparticles less than 10 nanometers thick along the c-axisdirection.

In other embodiments, the succession of shear strain events may beapplied until at least 90% of the graphite is exfoliated to form adistribution in the molten polymer phase of single- and multi-layergraphene nanoparticles less than 10 nanometers thick along the c-axisdirection.

In other embodiments, the succession of shear strain events may beapplied until at least 80% of the graphite is exfoliated to form adistribution in the molten polymer phase of single- and multi-layergraphene nanoparticles less than 10 nanometers thick along the c-axisdirection.

In other embodiments, the succession of shear strain events may beapplied until at least 75% of the graphite is exfoliated to form adistribution in the molten polymer phase of single- and multi-layergraphene nanoparticles less than 10 nanometers thick along the c-axisdirection.

In other embodiments, the succession of shear strain events may beapplied until at least 70% of the graphite is exfoliated to form adistribution in the molten polymer phase of single- and multi-layergraphene nanoparticles less than 10 nanometers thick along the c-axisdirection.

In other embodiments, the succession of shear strain events may beapplied until at least 60% of the graphite is exfoliated to form adistribution in the molten polymer phase of single- and multi-layergraphene nanoparticles less than 10 nanometers thick along the c-axisdirection.

In other embodiments, the graphite may be doped with other elements tomodify the surface chemistry of the exfoliated graphene nanoparticles.The graphite is expanded graphite.

In other embodiments, the surface chemistry or nanostructure of thedispersed graphite may be modified to enhance bond strength with thepolymer matrix to increase strength and stiffness of the graphenecomposite.

In other embodiments, directional alignment of the graphenenanoparticles is used to obtain one-, two- or three-dimensionalreinforcement of the polymer matrix phase.

In another embodiment, a graphene-reinforced polymer matrix composite isformed according to the methods described herein. Thermoplastic polymercomposites are provided in which polymer chains are inter-molecularlycross-linked by torn single- and multi-layer graphene sheets by means ofcovalent bonding sites exposed on the torn graphene sheet edges.

In certain embodiments, the thermoplastic polymer of thegraphene-reinforced polymer matrix composite is an aromatic polymer, asdefined above.

In other embodiments, the graphene-reinforced polymer matrix compositeconsists of graphite cross-linked with polymers selected from the groupconsisting of polyetheretherketone (PEEK), polyetherketone (PEK),polyphenylene sulfide (PPS), polyethylene sulfide (PES), polyetherimide(PEI), polyvinylidene fluoride (PVDF), polycarbonate (PC), polyphenyleneether, aromatic thermoplastic polyesters, thermoplastic polyimides,liquid crystal polymers, thermoplastic elastomers, polyethylene,polypropylene, polystyrene (PS), acrylics, such aspolymethylmethacrylate (PMMA), polyacrylonitrile (PAN), acrylonitrilebutadiene styrene (ABS), and the like, ultra-high-molecular-weightpolyethylene (UHMWPE), polytetrafluoroethylene (PTFE/Teflon®),polyamides (PA), such as nylons, polyphenylene oxide (PPO),polyoxymethylene plastic (POM/Acetal), polyaryletherketones,polyvinylchloride (PVC), mixtures thereof, and the like.

In other embodiments, the graphene-reinforced polymer matrix compositeconsists of graphite cross-linked with polyetheretherketone (PEEK).Sulfonated PEEK can also be cross-linked. PEEK that is cross-linked inthis manner will have very high specific properties and is suitable forautomotive, aviation and aerospace uses. The present invention thereforealso includes automotive, aircraft and aerospace parts formed from thecross-linked PEEK of the present invention, which can replace heaviermetal parts without a loss of mechanical or high temperature properties.For example, cross-linked PEEK can be used in engine components such aspistons, valves, cam shafts, turbochargers and the like because of itshigh melting point and creep resistance. Forming the rotating portionsof the turbine and compressor parts of a turbocharger from thecross-linked PEEK of the present invention will reduce turbocharger lagbecause of the resulting weight reduction. Other advantages are obtainedby forming the rotating portions of the turbine and compressor of jetengines from the cross-linked PEEK of the present invention.

EXAMPLES

The present invention is further illustrated by the following examples,which should not be construed as limiting in any way. While someembodiments have been illustrated and described, it should be understoodthat changes and modifications can be made therein in accordance withordinary skill in the art without departing from the invention in itsbroader aspects as defined in the following claims.

In one embodiment, a small scale extension mixer with a 10-gram capacitywas used to compound 2% of SMG with Udel P-1700 Polysulfone (PSU) at332° C. (630° F.) and under vacuum for 3, 30, and 90 minutes. The methodis described below. Samples collected for characterization after eachlength of time are referred to as 3G-PMC, 30G-PMC, 90G-PMC.

-   -   1. 9.8 grams of PSU were added to the mixer and allowed to        become molten.    -   2. 0.2 grams of SMG were added to the molten PSU and mixed.    -   3. After 3 minutes of mixing time, 3 grams of the G-PMC were        extruded out of the mixer and collected for characterization.    -   4. 3 grams of 2% SMG in PSU was added to the mixer and mixed.    -   5. After 30 minutes of mixing time, 3 grams of the G-PMC were        extruded out of the mixer and collected for characterization.    -   6. 3 grams of 2% SMG in PSU was added to the mixer and mixed.    -   7. After 90 minutes of mixing time, 3 grams of the G-PMC were        extruded out of the mixer and collected for characterization.

Morphology Analysis

A Zeiss Sigma Field Emission Scanning Electron Microscope (FESEM) withOxford EDS was used to determine the degree of mechanical exfoliation ofgraphite into multi-layer graphene or graphene nanoparticles and thethickness of these particles. An accelerating voltage of 3 kV andworking distance of approximately 8.5 mm was used during viewing. Priorto viewing, specimens from each sample of 3G-PMC, 30G-PMC, and 90G-PMCwere notched, cryogenically fractured to produce a flat fracturesurface, placed under vacuum for at least 24 hours, gold coated, andstored under vacuum.

X-Ray Diffraction Analysis (XRD)

XRD analysis on each sample of 3G-PMC, 30G-PMC, and 90G-PMC includesfour steps: (1) sample preparation, (2) diffraction pattern acquisition,(3) profile fitting, and (4) out-of-plane (D) crystallite sizescalculation according to the Debye-Scherrer equation.

-   -   1. The samples for XRD analysis were prepared by pressing thin        films of each sample 3G-PMC, 30G-PMC, and 90G-PMC at 230° C. and        5,500 psi over a 2 minute time period. Each sample was        positioned between aluminum sheets prior to pressing using a        Carver Uniaxial Press with heated platens.    -   2. Diffraction patterns of the pressed films were acquired using        a Philips XPert powder Diffractometer with sample changer        (Xpert) at 40 kV and 45 mA with an incident slit thickness of        0.3 mm from 4°-70° 2θ and a step size of 0.02° 2θ.    -   3. Diffraction patterns were uploaded into WinPLOTR Powder        diffraction graphics tool, without background editing or profile        adjustments prior to peak fitting. Single peak fitting was        applied at a 2θ range of 26°-27.5°, using a pseudo-Voigt        function and taking into account a global FWHM, global eta        (proportion of Lorentz), and linear background. Single peak        fitting of the profile provides the full width at half maximum        (FWHM) of the relevant peak.

The average out-of-plane crystallite size (D) (sometimes referred to asalong the c-axis, and proportional to the number of graphene layerswhich are stacked) is calculated using the Debye-Scherrer Equation andthe (002) FWHM values, for which X is the X-ray wavelength, coefficientK=0.89, β is the FWHM in radians, and θ is the diffraction angle. Thed-spacing is also calculated.

$\begin{matrix}{D = \frac{K\; \lambda}{\beta \; \cos \; \theta}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

Morphology Results

The morphology of each sample, 3G-PMC, 30G-PMC, and 90G-PMC, at threedifferent scales (magnification) is shown in FIG. 1. In (a-c), a 20 μmscale and 1,000× magnification shows good distribution of multi-layergraphene or graphene within the PSU matrix at each mixing time. In(d-f), a 1 μm scale and 10,000× magnification and (g-i), a 1 μm scaleand 50,000× magnification shows mechanically exfoliated graphite withinthe PSU matrix. In (d-i), micro-folding of the multi-layer graphene orgraphene is evident, as well as good bonding between the graphenenanoparticles and the polymer matrix.

The 90G-PMC sample, the sample mixed for the longest time and exposed tothe most repetitive shearing, exhibits superior mechanical exfoliationand the smallest crystal size. As shown in FIG. 2, mechanicalexfoliation has reduced the graphene nanoparticle thickness in the90G-PMC sample to 8.29 nm.

X-Ray Diffraction Results

The Debye-Scherrer equation was applied to the FWHM and d-spacingresults obtained from the X-ray diffraction patterns for 3G-PMC,30G-PMC, and 90G-PMC to provide the crystal thickness (D) of themulti-layer graphene or graphene nanoparticles. The XRD results andcrystal thickness appear in Table 1. For the 3G-PMC, 30G-PMC, and90G-PMC samples, the crystal thickness is 40 nm, 31 nm, and 23 nm; theFWHM is 0.202°, 0.257°, and 0.353°; and the d-spacing is 3.361 nm, 3.353nm, and 3.387 nm, respectively. The FWHM increases with mixing time, andcrystal thickness decreases with mixing time, which indicates thatmechanical exfoliation of the graphite to multi-layer graphene orgraphene is occurring and is enhanced over longer mixing times. Thedecrease in crystal size is a function of FWHM.

TABLE 1 Debye-Scherrer Equation applied to the average XRD results fromeach 2% Graphite Exfoliated in PSU sample mixed for 3 min, 30 min, and90 min Mixing Average D - Crystal Time (d 002) FWHM Thickness (nm) AlongSample (min) (nm) (degrees) c-Axis Direction 3G-PMC 3 0.3361 0.202 4030G-PMC 30 0.3353 0.257 31 90G-PMC 90 0.3387 0.353 23

Graphene Modification

Mechanical exfoliation of the graphite into multi-layer graphene orgraphene as a result of the repetitive shear strain action in thepolymer processing equipment generates dangling primary and secondarybonds that provide the opportunity for various chemical reactions tooccur, which can be exploited to obtain property enhancement of theG-PMC. This represents an advance over prior art conventional methodsforming graphene oxides, where the dangling primary and secondary bondscovalently bond with oxygen, which typically remain in these positionseven after the graphene oxide is reduced.

For example, chemical reactions that covalently attach these danglingbonds from the multi-layer graphene or graphene nanoparticles to thepolymer matrix would provide superior mechanical properties of theG-PMC. Alternatively, electrical conductivity may be enhanced bychemically linking appropriate band gap materials at the graphenenano-particle edges or by coordinating with conductive metals such asgold, silver, copper, and the like. The graphene-reinforced polymer maythen be added to polymers or other compositions to provide or increaseelectrical conductivity. The bonds may also be coordinated to metals,such as platinum and palladium, to provide a catalyst, with thegraphene-reinforced polymer serving as a catalyst support. Other formsof functionalized graphene are disclosed in U.S. Pat. No. 8,096,353, thedisclosure of which is incorporated herein by reference.

The method of the present invention is particularly advantageous becausein situ functionalization reactions may be performed during theexfoliation process via one-pot reactive compounding.

The graphene-reinforced polymers may be used as electrodes forlightweight batteries. Other uses include composite boat hulls,aircraft, aerospace systems, transportation vehicles, lightweight armor(vehicular or personnel armor), pressure vessels, reactor chambers,spray coatings, polymer powders for 3-D printing, transparent electrodesfor electronic device touch screens, and the like. Addition of 1-2 wt %graphene to a polymer matrix imparts electrical conductivity, whilemaintaining optical transparency, thus enabling applications in solarpanels, flat-panel displays, and for static-discharge control inhospitals.

Mechanical exfoliation successfully converted 2% graphite melt-blendedwith PSU into a G-PMC using a repetitive shearing action in the SmallScale Extension Mixer by Randcastle Extrusion Systems, Inc.(“Randcastle”). Results may be improved by machine modification toincrease shear; for example, by using a larger diameter mixing elementto increase rotational speed and/or by minimizing the spacing betweenthe mixing element and the cylinder wall.

Modified Randcastle Extrusion System's Small Scale Extension Mixer:

The design of the existing small batch mixer may be modified to providehigher shear rate, which in turn provides superior mechanicalexfoliation of graphite within the polymer matrix. The shear rate, {dotover (γ)}, is calculated according to Equation 1, where r is the toolingradius and Ar is the clearance for compounding. Machine modificationsare listed in Table 2, along with the maximum achievable shear rate. Thenewly designed mixer has a maximum shear rate 22 times that of thecurrent mixer, which will provide enhanced mechanical exfoliation ofgraphite within a polymer matrix at shorter lengths of time. In otherwords, the crystal size, D, may be reduced to smaller dimensions in amore efficient length of time.

TABLE 2 Modifications of the Randcastle Extrusion System's Small ScaleExtension Mixer to provide enhanced mechanical exfoliation CurrentImproved Randcastle Randcastle Mixer Mixer Tooling Radius (inches) 0.5 1Clearance for Compounding, Δr (in) 0.04 0.01 Maximum RPM 100 360 MaximumShear Strain Rate (sec⁻¹) 133 2900

Modified Single Screw Extrusion:

Randcastle has made modifications to the extruder screw that will betterenable mechanical exfoliation of the graphite into multi-layer grapheneor graphene in a polymer matrix to fabricate a G-PMC.

Materials

Raw graphite was extracted from the ground, crushed to powder, and floatseparated to obtain Separated Mineral Graphite (“SMG”).

PEEK has a specific gravity of 1.3, a melt flow of 3 g/10 min (400° C.,2.16 kg), a glass transition temperature at 150° C., and a melting pointat 340° C. The tensile modulus and strength are 3.5 GPa and 95 MPa,respectively. Prior to the creation of the xG-PMC in this example, SMGand PEEK were dried for approximately 12 hours at 100° C. and 150° C.,respectively.

In this example, SMG was blended with PEEK using a Randcastlemicro-batch mixer with a 10-gram capacity at 360° C. (680° F.) and 100RPM under a nitrogen blanket, according to the following steps:

-   -   PEEK_(—)3—To create a control sample, 10 grams of PEEK was added        to the mixer. After three minutes of mixing time, the port was        opened to allow PEEK to flow out as extrudate and 2.6 grams were        extruded out until no more material was able to flow.    -   SMG-PEEK_(—)3—To create a weight composition ratio of 2-98%        SMG-PEEK, 2.4 g of PEEK and 0.2 g of SMG were added to the        mixer. After three minutes of mixing time, the port was opened        to allow G-PMC to flow out as extrudate and 1.96 g were extruded        out until no more material was able to flow.    -   SMG-PEEK_(—)30—To maintain the 2-98 wt % composition ratio, 1.92        g of PEEK and 0.04 g of SMG were added to the mixer. After 30        minutes of mixing time, the port was opened to allow G-PMC to        flow out as extrudate and 0.94 g were extruded out until no more        material was able to flow.    -   SMG-PEEK_(—)90—To maintain the 2-98 wt % composition ratio, 0.92        g of PEEK and 0.02 g of SMG were added to the mixer. After 90        minutes of mixing time, the port was opened to allow G-PMC to        flow out as extrudate, however, no more material was able to        flow.

The experiment was terminated and the mixer opened. Under visualobservation, the G-PMC did not appear as a standard molten polymer, butrather was in a rubber-like, fibrous form.

In this next example, SMG and PEEK were processed in a Randcastlemicro-batch mixer with a 100-gram capacity at 360° C. (680° F.) and 30RPM under a nitrogen blanket, according to the following steps:

-   -   PEEK_(—)90—To create a control sample, 100 g of PEEK was added        to the mixer. After 90 minutes of mixing time, the port was        opened to allow PEEK to flow out as extrudate and 28.5 g were        extruded out until no more material was able to flow.    -   SMG-PEEK_(—)25—To create a weight composition ratio of 2-98%        SMG-PEEK, 98 g of PEEK and 2 g of SMG were added to the mixer.        After 25 minutes, of mixing time, the port was opened to allow        G-PMC to flow out as extrudate and 5.1 g were extruded out until        no more material was able to flow.

Characterization

The samples used for characterization appear in Table 3, as follows:

TABLE 3 Samples Used for Characterization Batch Mixer Graph SampleDescription (Capacity) Color PEEK_3 Control mixed for 3 minutes 10 gGreen PEEK_90 Control mixed for 90 minutes 100 g  Purple SMG-PEEK_3Components mixed for 10 g Orange 3 minutes SMG-PEEK_30 Components mixedfor 10 g Blue 30 minutes SMG-PEEK_90 Components mixed for 10 g Red 90minutes

Morphology

The morphology of the xG-PMC was examined using a Zeiss Sigma FieldEmission Scanning Electron Microscope (“FESEM”) with Oxford EDS. Anaccelerating voltage of 3 kV and working distance of approximately 8.5mm was used during viewing. Prior to viewing, specimens were notched,cryogenically fractured to produce a flat fracture surface, placed undervacuum for at least 24 hours, gold coated, and stored under vacuum. Asillustrated in FIG. 3, the morphology of SMG-PEEK_(—)90 is shown in (a)10 μm scale and 1,000 magnification (b) 10 μm scale and 5,000magnification, (c) 1 μm scale and 10,000 magnification, and (d) 1 μmscale and 50,000 magnification.

Thermal Analysis

The thermal properties of the samples were characterized using a TAInstruments Q1000 Differential Scanning calorimeter (DSC). Each samplewas subject to a heat/cool/heat cycle from 0-400° C. at 10° C./min. Theglass transition temperature (Tg) and melting temperature (Tm) for theinitial heat scan are illustrated in FIG. 3. The Tg increases from 152°C. for PEEK_(—)3 to 154 for SMG-PEEK_(—)90, however, this increase isnot significant. The Tm is consistent for samples PEEK_(—)3,SMG-PEEK_(—)3, and SMG-PEEK_(—)30 at almost 338° C. but decreasessignificantly to 331.7° C. for SMG-PEEK_(—)90. The delta H is similarfor samples PEEK_(—)3, SMG-PEEK_(—)3, and SMG-PEEK_(—)30, and variesbetween the initial, cool, and reheat scans, and ranges between 116-140J/g. However, the delta H for SMG-PEEK_(—)90 is much lower andconsistent at approximately 100 J/g for the initial, cool, and reheatscans. The observable difference in the heat of fusion of PEEK for theSMG-PEEK_(—)90 sample, as compared with the other samples, indicates amajor difference in the morphology. Furthermore, the constant heat offusion between the initial, cool, and reheat scans of the SMG-PEEK_(—)90sample supports the existence of cross links between the graphene andPEEK matrix.

Parallel Plate Rheology

A frequency sweep from 100-0.01 Hz at 1.0% strain and at a temperatureof 360° C. was performed using a TA Instruments AR 2000 in parallelplate mode. Samples SMG-PEEK_(—)30, SMG-PEEK_(—)3, and PEEK_(—)3 weretested. The G′ and G″ and the tan delta for samples SMG-PEEK_(—)30,SMG-PEEK_(—)3, and PEEK_(—)3 were recorded. Tan delta is equal to theG″/G′. This rheology data provides information regarding the morphologyof the sample, according to Table 4, as shown below. The sol/geltransition point, or “gel point”, of a thermoset resin occurs when tandelta=1, or rather when G′=G″. For samples SMG-PEEK_(—)3 and PEEK_(—)3,the G″ is greater than the G′, indicating liquid-like behavior.Contrastingly for sample SMG-PEEK_(—)30, the G′ is greater than G″,indicating more elastic-like or solid-like behavior. Furthermore, tandelta is less than 1 and remains nearly constant across the entirefrequency range for SMG-PEEK_(—)30, indicating that SMG-PEEK_(—)30 hasundergone some degree of cross-linking

TABLE 4 Rheology data and the sol/gel transition point Shear and SampleState Morphology Tan δ Loss Moduli Behavior Liquid “Sol State” >1 G″ >G′  PEEK_3 state SMG-PEEK_3 Gel point Cross linking begins =1 G′ = G″Gel State Solid State Sample <1 G′ > G″ SMG-PEEK_30 contains cross-links

Dissolution

Lightly gelled thermosetting resins when placed in solvents swellthrough imbibition to a degree depending on the solvent and thestructure of the polymer. The original shape is preserved, and theswollen gel exhibits elastic rather than plastic properties.Cross-linking in thermoplastic polymers is commonly accomplished by 1)peroxides, 2) a grafted silane process cross-linked by water, 3)electron beam radiation, and 4) UV light.

In this example, cross-linking was induced between SMG and PEEK during amechanical exfoliation process due to the cleavage of graphene flakesthat results in dangling free radicals. To confirm the presence ofcross-linking in the SMG-PEEK XG-PMC, a dissolution method was used byplacing neat PEEK, PEEK_(—)3, PEEK_(—)90, SMG-PEEK_(—)3, SMG-PEEK_(—)30,and SMG-PEEK_(—)90 samples in sulfuric acid, according to the followingsteps.

-   -   A 10 mg specimen from each sample was prepared;    -   Each specimen was placed in a test tube with 20 mL of 95-98% w/w        sulfuric acid (A300S500 Fisher Scientific);    -   The solution was shaken for 5 minutes;    -   Each test tube was capped with Teflon® tape to form a seal;    -   Photographs of each sample were taken at times 0, 24, 48, and 72        hours.

Upon visual observation, the PEEK samples all dissolve within thesulfuric acid before 24 hours, and the SMG-PEEK_(—)90 sample is the onlyone that remains in the sulfuric acid after 72 hours. The SMG-PEEK_(—)90sample was cross-linked and swelled when placed in the solvent similarto a thermoset resin. The SMG-PEEK_(—)30 sample remained in the sulfuricacid after 24 hours but dissolved before 48 hours. SMG-PEEK_(—)30required further testing to determine if cross-linking was induced,since the other data suggests that SMG-PEEK_(—)30 was cross-linked.

The foregoing examples and description of the preferred embodimentsshould be taken as illustrating, rather than as limiting the presentinvention as defined by the claims. As will be readily appreciated,numerous variations and combinations of the features set forth above canbe utilized without departing from the present invention as set forth inthe claims. Such variations are not regarded as a departure from thespirit and scope of the invention, and all such variations are intendedto be included within the scope of the following claims.

1. A method for forming a graphene-reinforced polymer matrix composite,comprising: (a) distributing graphite microparticles into a moltenthermoplastic polymer phase comprising one or more molten thermoplasticpolymers; and (b) applying a succession of shear strain events to themolten polymer phase so that said molten polymer phase exfoliates thegraphene successively with each event, until tearing of exfoliatedmultilayer graphene sheets occurs and produces reactive edges on saidmultilayer sheets that react with and cross-link said one or morethermoplastic polymers; wherein said one or more thermoplastic polymersare selected from the group consisting of thermoplastic polymers subjectto UV degradation.
 2. The method of claim 1, wherein at least one ofsaid one or more thermoplastic polymers is an aromatic polymer.
 3. Themethod of claim 2, wherein said aromatic polymer comprises phenylgroups, optionally substituted, in either the backbone or assubstituents.
 4. The method of claim 3, wherein the optionallysubstituted phenyl groups are contained within the polymer backbone asoptionally substituted phenylene groups.
 5. The method of claim 3,wherein the optionally substituted phenyl groups are substituents on thepolymer.
 6. The method of claim 1, wherein said one or morethermoplastic polymers are selected from the group consisting ofpolyetheretherketone (PEEK), polyetherketone (PEK), polyphenylenesulfide (PPS), polyethylene sulfide (PES), polyetherimide (PEI),polyvinylidene fluoride (PVDF), polycarbonate (PC), polyphenylene ether,aromatic thermoplastic polyesters, thermoplastic polyimides, liquidcrystal polymers, thermoplastic elastomers, polyethylene, polypropylene,polystyrene (PS), polymethylmethacrylate (PMMA), polyacrylonitrile(PAN), ultra-high-molecular-weight polyethylene (UHMWPE),polytetrafluoroethylene (PTFE), acrylonitrile butadiene styrene (ABS),polyamides (PA), polyphenylene oxide (PPO), polyoxy-methylene plastic(POM/Acetal), polyimides, polyaryletherketones, polyvinylchloride (PVC),acrylics, and mixtures thereof.
 7. A method for forming a high-strengthgraphene-reinforced polymer matrix composite, comprising: (a) formingthe composite of claim 1 into cross-linked polymer particles; and (b)distributing the polymer particles into another non-cross-linked moltenhost thermoplastic matrix polymer.
 8. The method of claim 1, whereinsaid molten thermoplastic polymer phase comprises two or more moltenthermoplastic polymers.
 9. The method of claim 1, wherein the graphiteparticles are prepared by crushing and grinding a graphite-containingmineral to millimeter-sized dimensions, followed by milling to amicron-sized particle mixture.
 10. The method of claim 9, wherein thegraphite particles are extracted from the micron-sized particle mixtureby a flotation method.
 11. The method of claim 6, wherein said polymeris polyetheretherketone (PEEK).
 12. The method of claim 1, wherein thegraphite is expanded graphite.
 13. A graphene-reinforced polymer matrixcomposite prepared according to the method of claim
 1. 14. Thegraphene-reinforced polymer matrix composite of claim 13, wherein saidpolymer is polyetheretherketone.
 15. A high strength graphene-reinforcedpolymer matrix composite prepared according to the method of claim 7.16. The graphene-reinforced polymer matrix composite of claim 15,wherein said polymer is polyetheretherketone.
 17. A thermoplasticpolymer composite comprising thermoplastic polymer chainsinter-molecularly cross-linked by torn single- and/or multi-layergraphene sheets having carbon atoms with reactive bonding sites on thetorn edges of said sheets.
 18. The thermoplastic polymer composite ofclaim 17, wherein said thermoplastic polymers are selected from thegroup consisting of polyetheretherketone (PEEK), polyether-ketone (PEK),polyphenylene sulfide (PPS), polyethylene sulfide (PES), polyetherimide(PEI), polyvinylidene fluoride (PVDF), polycarbonate (PC), polyphenyleneether, aromatic thermoplastic polyesters, thermoplastic polyimides,liquid crystal polymers, thermoplastic elastomers, polyethylene,polypropylene, polystyrene (PS), polymethylmethacrylate (PMMA),polyacrylonitrile (PAN), ultra-high-molecular-weight polyethylene(UHMWPE), polytetra-fluoroethylene (PTFE), acrylonitrile butadienestyrene (ABS), polyamides (PA), poly-phenylene oxide (PPO),polyoxymethylene plastic (POM/Acetal), polyimides,polyarylether-ketones, polyvinylchloride (PVC), acrylics, and mixturesthereof.
 19. An automotive, aircraft or aerospace part formed from thecomposite of claim
 17. 20. The part of claim 19, wherein said part is anengine part.
 21. Graphene cross-linked polymer particles formed from thecomposite of claim
 17. 22. A polymer composition comprising a hostthermoplastic polymer and the graphene cross-linked polymer particles ofclaim 21 dispersed therein.
 23. The polymer composition of claim 22,wherein said host thermoplastic polymer is selected from the groupconsisting of polyetheretherketone (PEEK), polyether-ketone (PEK),polysulfones (PS), polyphenylene sulfide (PPS), polyethylene sulfide(PES), polyetherimide (PEI), polyvinylidene fluoride (PVDF),polycarbonate (PC), polyphenylene ether, aromatic thermoplasticpolyesters, thermoplastic polyimides, liquid crystal polymers,thermoplastic elastomers, polyethylene, polypropylene, polystyrene (PS),polymethyl-methacrylate (PMMA), polyacrylonitrile (PAN),ultra-high-molecular-weight polyethylene (UHMWPE),polytetra-fluoroethylene (PTFE), acrylonitrile butadiene styrene (ABS),polyamides (PA), poly-phenylene oxide (PPO), polyoxymethylene plastic(POM/Acetal), polyimides, polyarylether-ketones, polyvinylchloride(PVC), acrylics, and mixtures thereof.
 24. An automotive, aircraft oraerospace part formed from the polymer composition of claim 22.